This invention relates generally to automatic control systems. More specifically, it relates to electric control systems wherein the control signals utilized are combined functions of a proportional signal and the integral and derivative thereof.
Automatic control systems are known in the art. Such systems receive electrical signals representative of a condition of an element to be controlled and compare that signal with a reference to produce an error signal. A particular application of such control systems has been as governors controlling the speed (RPM) of internal combustion engines.
In many applications it is desired to control engine RPM to maintain constant speed in spite of variations of engine load. It is desirable to provide a system wherein the transient response, that is, the response time of the engine and the control system, due to a change in operating conditions, is minimized. This avoids over speed, hunting and instability conditions potentially damaging to the system.
An automatic control system having the characteristics desired for the aforementioned uses is disclosed in co-pending U.S. Pat. application Ser. No. 538,133 filed Jan. 2, 1975, and assigned to the present assignee. In that application, which is hereby incorporated by reference, an automatic control system is described in which engine speed is sensed by a magnetic pickup coil. After wave shaping and amplification the wave form is applied through an isolating switch arrangement to the input of an operational amplifier. This amplifier compares the average DC level against a reference to generate an error signal. The error signal is then differentiated and integrated with respect to time. A signal proportional to the error signal, its integral and its derivative are combined to produce a control signal for operating an actuator device which may control a fuel valve or throttle linkage. By use of a feedback element associated with the actuator, accurate control over the device is obtained.
In order for safe operation of the controlled device, it is often necessary to provide one or more external condition limit circuits. Such circuits generate an overriding control signal in the event that a detected condition occurs. Such conditions include excessive engine temperature, excessive smoke, improper manifold pressure, and the like. When such a condition occurs, it is desired to override the automatic control circuit regardless of the normal operating conditions, such as engine speed, and to lower the operating speed of the engine or shut it down altogether depending upon the considerations involved.
When a limit control overrides the automatic control system, the integrator circuit of the control system is "fooled" by the continued operation of the engine at a value other than that specified by the automatic control. This causes the integrator to begin operating in a manner disadvantageous to precise control when the controlling external limit is removed.
If no provision is made for offsetting the integrator during operation of a limit, then for a considerably long period of time after the external limit is removed the integrator will cause the automatic control system to operate in an improper manner as, for example, hunting for the set point. This is because the integrator output is utilized as one measure of the amount of error between the set speed and the actual operating speed. Therefore, if the integrator is considerably away from its proper value, the circuit will not provide as precise control as is otherwise obtainable.
It is accordingly an object of the invention to provide an integrator offset circuit which will prevent the integrator from saturating during a period of operation in which an external limit overrides the automatic control system.
It is another object of the invention to provide an integrator offset circuit which, responsive to the presence of an external control condition, maintains an integrator circuit in a desired operating region for the duration of the applied external limit signal.
A further object of the invention is to provide an integrator offset circuit for an automatic control system.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.